Cathode-ray tube electrode



1953 R. w. SHAWFRANK CATHODE-RAY TUBE ELECTRODE Filed July 3, 195

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Patented Jan. 27, 1953 CATHODE-RAY TUBE ELECTRODE Robert W. Shawfrank, Chicago, Ill., assignor to The Rauland Corporation, a corporation of Illinois Application July 3, 1951, Serial No. 234,920

2 Claims.

This invention relates to image-reproducing devices and more particularly to cathode-ray tubes of the electrostatically focused type.

It is a primary object of the invention to provide a new and improved focusing electrode for a cathode-ray tube of the electrostatically focused type.

It is a further object of the invention to provide a new and improved focusing electrode for a unipotential electrostatic focusing lens system of the low-potential type, i. e., of the type in which the focusing electrode is adapted to be operated at a potential of or less of the final anode voltage, so that no operating potential intermediate the B-supply voltage of the associated receiver apparatus and the final anode voltage is required.

Fundamentally, a unipotential electrostatic focusing lens system comprises three electrodes, the outermost two of which are operated at a common high positive potential, while the intermediate one is operated at a lower potential to provide the desired electrostatic field distribution comprising convergent and divergent lens components and havin a net convergent effect In accordance with the present invention, the intermediate electrode, hereinafter termed the lens electrode, comprises a metal disc having an aperture, and a metal inset having a cylindrical portion within the aperture and having an outwardly rolled rim at each end overlapping and terminally abutting the metal disc.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, maybest be understood, however, by ref erence to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals indicate like elements, and in which:

Figure l is a fragmentary side elevation, partly in cross-section and partly cut away, of an imagereproducing device employing a focusing electrode constructed in accordance with the present invention;

Figure 2 is a plan view of a focusing electrode embodying the invention;

Figure 3 is a side elevation of the electrode illustrated in Figure 2;

Figure 4 is a partial cross-section, taken along the line 4-t of Figure 2, and

Figure 5 is a schematic diagram of a television receiver embodying the image-reproducing device of Figure l.

The image-reproducing device of Figure 1 comprises a fluorescent screen It affixed to the glass target portion 5 i of a cathode-ray tube envelope which also comprises a glass neck portion l2 enclosing an electron gun and an electrostatic focusin system. The electron gun comprises a cathode IS, a control electrode l4, and first and second accelerating electrodes l5 and it respectively, A diaphragm l1 having a central aperture H3 is disposed across the outlet end of second accelerating electrode I t, and aperture 18 is symmetrically centered With respect to the tube axis AA perpendicular to the center of the fluorescent screen iil. Second accelerating electrode 16 is laterally offset from first accelerating electrode E5 to provide a transverse electrostaticdeflection field in the region between these two electrodes, and the entire electron gun structure is tilted with respect to the tube axis A-A by an angle 0.

An electrostatic focusing system of the unipotential lens type is disposed between the electron gun and the fluorescent screen. The focusing system comprises the outlet end of second accelerating electrode 56 including diaphragm H, a lens electrode 19, and an additional electrode 20 which are all coaxially mounted with respect to the tube axis A--A. Diaphragm I7 and additional electrode 29 are maintained at a common operating potential by means of connecting strips 2!, while lens electrode l 9 is provided with a separate lead 22 extending through the base 23 of the tube. Additional electrode 20 is further maintained at a common potential with a conductive coating 24, of colloidal graphite such as aquada-g or the like, on the inner wall of the tube envelope, by means of metal spacer springs 25. Conductive coating 24 extends toward the base only as far as electrode 20 to avoid undesirable spark discharge between that coating and lens electrode l9, and lead 22 may be provided with an insulating glass bead (not shown) to prevent spark discharge to electrode H5.

For convenience, electrodes l4, l5, l6, l9 and 26 may be termed grids and may be designated by number starting with control electrode M as the first grid and progressing in the direction of beam travel to additional electrode 20 which is the fifth grid. All five grids are supported in predetermined mutually spaced relation by means of a pair of glass pillars 26, of which only one is shown, in a manner which will be apparent to those skilled in the art. Separate leads for grids l, 2 and extend through the base 23 of the tube, as do the supply leads for the cathode l3 and its associated heater element (not shown). Operating voltage for the conductive coating 24, and therefore for the third and fifth grids, may be supplied byv means of a conventional contact button if the envelope is of the all-glass type, or directly to the metal cone member if the tube is of the glass-metal variety.

An external permanent magnet 2'1, supported in a spring clamp 28 which fits snugly around the neck of the tube and is movable both axially and rotationall is provided to develop a magnetic held within the tube to provide separation of the negative ions from the electron beam. Moreover, a fluorescent coating 23 on the outer surface of the second accelerating electrode iii (grid 3) is provided for facilitating alignment of iontrap beam-bender magnet 21.

The tube is evacuated, sealed and based in accordance with well-known procedures which require no further explanation, and suitable getters 30 are supported from grid to absorb residual gases after evacuation.

In operation, a mixed beam of electrons and negative ions originating at cathode I3 is projected through the aperture in first accelerating electrode l5. When the mixed beam emerges from grid 2, it encounters an electrostatic field having a transverse component due to the lateral offset of grid 3 with respect to grid 2. Consequently, electrons and ions are both deflected upwardly in the view of Figure l. The magnetic field imposed by beam-bender magnet 27 serves to deflect the electrons in a downward direction as viewed in Figure 1 without substantially affecting the path taken by the negative ions. Thus, when beam-bender magnet 2! is accurately adjusted, the beam of electrons is projected centrally through aperture I8 of diaphragm ll in a direction along the tube axis A.A, while the negative ions are intercepted by the metallic portions of grid 3 and diaphragm ll. The ion-trap mechanism is disclosed and claimed in the copending application of Willis E. Phillips et al., Serial No. 156,746, filed April 19, 1950, for Electron Gun for Cathode-Ray Tubes, now U. S. Patent No. 2,596,508 issued May 13, 1952, and assigned to the present assignee.

The axially directed electron beam is subjected to the focusing action of the electrostatic fields produced by diaphragm I'I, lens electrode l9, and the fifth grid 2!} which together constitute a unipotential electrostatic focusing lens system. The general construction and operation of lenses of this type are well understood by those skilled in the art as indicated by an article entitled Measured properties of strong unipotential electron lenses by G. Liebmann, Proceedings of the Physical Society, section B, volume 62, part 4, pages 213-228 (April 1, 1949).

The required operating potential difference between the lens electrode (grid 4) and the other electrodes of the focusing system (grids 3 and 5) is determined by the dimensions of and the spacing between the electrodes constituting the unipotential lens. Although the relationships are not necessarily linear, the required focusing potential difference varies directly with the length and inversely with the diameter of grid 4, and inversely with the separation between grid 4 and grids 3 and 5. Certain limitations on these parameters are imposed by practical considerations; if the diameter of grid 4 is made too small, excessive spherical aberration is encountered, and if the separation between grids 3 and d is made too great, the deflecting influence exerted by the asymmetrical electrostatic field established between lead wire 22 and grid 3 becomes objectionable.

Unipotential electrostatic lens systems have previously been employed in cathode-ray tubes. Such lens systems have been found quite satisfactory and readily adaptable to mass production techniques when the electron gun and the focusing system are coaxial and the path of the beam is restricted to the tube axis during its entire progress from the cathode through the focusing system. However, according to present commercial practice, nearly all television picture tubes are provided with an ion-trap mechanism of one sort or another for removing negative ions from the electron beam in order to avoid deterioration of the fluorescent screen. Such types of ion-trap arrangements as are commonly employed provide ion separation by subjecting the mixed beam to opposed electrostatic and magnetic fields; both electrons and ions are transversely deflected by the electrostatic field, while the electrons only are substantially deflected in the opposite direction by the impressed magnetic field. It is apparent that the practical requirement for ion trapping results in a displacement of the electron beam from the tube axis, and it is necessary that the beam be directed to the axis before it enters the focusing system. The accuracy with which this is accomplished is of importance when magnetic focusing is employed or when a unipotential electrostatic lens system is employed with a focusing voltage source which may be varied over a relatively wide range, but slight misalignment may be compensated by varying the focusing voltage. However, the ion trap alignment becomes much more critical when satisfactory operation of the electrostatic focusing system is required within a narrow focusing-voltage range between cathode potential and the B- supply voltage of the associated apparatus, owing to the increased strength of the individual lens components constituting the unipotential focusing system.

In general, if the electron beam is inaccurately centered or approaches the focusing system in an angular manner, multiplicity of focus is encountered. The most troublesome form in which this manifests itself is that of astigmatism and/or coma. In order to obtain focusing comparable with that provided by magnetic focusing systems, while operating the lens electrode at or near cathode potential, it has been found essential that the electron beam be centrally directed along the axis in its passage through the focusing system. In other words, the ion trap must be precisely adjusted for satisfactory focusing with a low-potential electrostatic lens system. Such precise alignment of the ion trap is facilitated by providing a fluorescent coating 29 on the outer wall of second accelerating electrode I6. This fluorescent coating serves as an iontrap adjustment indicator; in practice, permanent magnet 27 is moved both axially and rotationally until the glow from fluorescent coating 29 is reduced to a minimum, thus indicating precise ion trap alignment. The fluorescent coating must of course be so situated that it is excited into fluorescence whenever the electron beam is entirely or partially intercepted by diaphragm I1, and the fluorescent glow must be visible, either directly or by reflection, through the transparent neck portion ii! of the tube envelope. Other possible locations for fluorescent coating 29 in.- clude the inner wall of neck portion l2 near the space between grids 2 and 3, the inner surface of diaphragm I1, and the surface of diaphragm 20 facing the cathode. The ion trap indicator is described and claimed in the copending applications of Constantin S. Szegho, Serial No. 134,725, filed December 23, 194-9, now U. Patent No. 2,564,737, issued August 21, 19-51, and of Constantin S. Szegho et Serial No. 162,905, filed May 19, 1950, now U. S. Patent No. 2,565,533, issued August 28, 1951, both entitled "Cathode- Ray Tube, and both assigned to the present asslgnee.

In order to obtain satisfactory focusing with the system shown in Figure 1, it is necessary to maintain rather stringent manufacturing tolerances with respect to the dimensions and spacings of the several electrodes constituting the focusing system. In addition, since grid 4 is to be operated at a potential substantially equal to that of the cathode, extremely high voltage gradients are produced between grid 4 and grids 3 and 5. In order to avoid undesirable corona effects and field emission, grids 3 and 5 are each provided with corona rings 3| and 32 in the form of rolled flanges of stainless steel or the like which are welded or otherwise secured to the respective electrodes, and grid 4 is constructed by rolling the two ends of a metal cylinder 33 over the edge of a large aperture in a metal disc 34, as hereinafter described in greater detail.

The corona rings 3| and 32 also perform an additional function in facilitating the maintenance of the required close manufacturing tolerances by mechanically reinforcing the circular flanges to which they are attached against warping or bending during the assembly of the elecstructions now employed in magnetically focused television picture tubes, the electron gun of the tube of Figure 1 has been modified in two further structural respects. In the first place, the angle 0 by which the entire gun is tilted with respect to the tube axis A-A is reduced from about 6 degrees to about 4% degrees, and the amount of lateral offset of grid 3 with respect to grid 2 is reduced from 1.9 mm. to 1.6 mm. In the second place,-the length of the tubular portion of grid 3 is reduced from about 38 millimeters to about millimeters.

These two parameters influence the performance ofthe electrostatic focusing system in the following manner. In order to obtain satisfactory focusing with the reduced focusing-voltage range, it is desirable to make the electron beam more nearly parallel as it enters the focusing system. This may be accomplished by locating the unipotential lens system nearer to the cathode and thereby decreasing its focal length; for this reason grid 3 is shortened. However, it is not possible to shorten grid 3 indefinitely because a certain minimum length is required to insure ion trapping at normal operating voltages. To oompensate for the impaired performance of the ion trap occasioned by shortening grid 3, the amount of lateral ofiset between grids 2 and 3 and the angle 0 by which the entire electron gun is tilted withrespect to the tube axis AA may each be increased; however, increasing the angle of gun tilt results in an increase'in the angle at which the beam enters the focusing system, resulting in greater astigmatism and/or coma, sothat from this point of View the angle of gun tilt should be made as small as possible consistent with the requirement for ion-trapping with a single beambender magnet. The condition represented by the relationships set forth in the preceding paragraph has been found to provide good focusing while retaining ion trapping at reasonable operating voltages. In this connection, it is observed that the particular type of offset ion trap employed in the tube of Figure 1 has the important advantage over other types of ion-trap gun, such as that employing coaxial electrodes with a slanted aperture between grids 2 and 3, that a greater angle of gun tilt may be employed for a given amount of lens distortion since the cathode is closer to the tube axis. Moreover, the offset type of ion trap is free of the characteristic elliptical distortion associated with the coaxial slanted-aperture type.

For best results, it has been found that the apertures in grids I, 2, 3 and 5 should be in marginally overlapping alignment in a direction parallel to the tube axis A-A. In other words, all

of these apertures should intercept an imaginary straight line parallel to reference axis A-A, and the apertures in grids l and 2 should intercept that line asymmetrically. Fulfillment of this condition is dependent upon the angle 0 by which theentire electron gun is tilted with respect to the tube axis, and also upon the length of the electron gun from the cathode to aperture l8 in diaphragm 11. If the angle 0 and/or the length of the gun is increased to such an extent that the apertures in grids I, 2, 3 and 5 are no longer in marginally overlapping alignment in a direction parallel to the tube axis, increased multiplicity of focus is encountered, and the performance of the focusing system is inferior. On the other hand, if the angle 0 is decreased so that the apertures are in complete coaxial alignment, ion trapping may no longer be conveniently accomplished with a single beam-bender magnet. I

In accordance with the present invention, grid 4 is constructed in the manner shown in detail in Figures 24. Specifically, grid 4 comprises a metal disc 34 having a circular aperture the diameter of which is large relative to the thickness of disc 34. Grid 4 also comprises a metal inset 33 having a central cylindrical portion 35 externally engaging the boundary of the aperture of disc 34 and provided at each end with outwardly rolled rims 36 and 3'! which overlap and terminally abut metal disc 34, as best illustrated in Figure 4. The diameter of the rolled-over ends of inset 33 is quite critical, the potential difference for optimum focus varying directly (though not necessarily linearly) with this dimension. Disc 34 is provided with a pair of tabs 38 which are adapted to be fused into glass support pillars 26 Figure 1) and with a lip 39 to which lead 22 (Figure l) is adapted to be connected. Grid 4, as well as each of the other electrodes, is constructed of nonmagnetic material to avoid undesirable distortion or suppression of the magnetic field induced by ion-trap magnet 21 and the magneticdefiection coil, as well as to permit the use of a beam-centering magnet if desired.

While inset 33 constitutes the effective part of the lens electrode with respect to the focusing action of the unipotential electron lens, disc 34 also has an important electrical role. The use of common support pillars for all five grids is desirable from the point of view of maintaining close mechanical tolerances, but the surfaces of the pillars opposite the gaps between grid 4 and grids 3 and 5 may acquire static charges tending to establish extraneous asymmetrical fields. Disc 34 effectively performs a shielding function by substantially preventing these extraneous fields from penetrating to the axis of the tube, thus avoiding asymmetrical spot distortion.

In the television receiver schematically illusrated in Figure 5, incoming composite television signals are received and separated into videosignal components and synchronizing-signal components by means of conventional receiver circuits 40 which may include a radio-frequency amplifier, an oscillator-converter, an intermediate-frequency amplifier, a. video detector, a video amplifier and a synchronizing-signal separator, as well as suitable circuits for reproducing the sound portion of the received signal. The detected composite video signal from receiver circuits 40 is applied between the control electrode I 4 and the grounded cathode l3 of an image-reproducing device 4l of the type shown in Figure 1. Synchronizing-signal components of the detected composite video signal are employed to drive a synchronizing system 42 of conventional construction which supplies the line-frequency and field-frequency deflection coils 43 and 44 with suitable scanning currents to control the scansions of the cathode-ray beam of device 4|.

A high-voltage power supply 45, which may also be of conventional construction, is employed to provide a suitable high operating voltage for the conductive coating 24 to which grids 3 and 5 are internally connected. Lens electrode [9 (grid 4) is connected to a variable tap 46 associated with a potentiometer resistor 4'! connected between the receiver DC voltage supply source, conventionally designated 28+, and ground. With the tube construction shown and described in connection with Figure 1, optimum focus of the cathode-ray beam is achieved when the minimum glow of fluorescent coating 29 indicates precise ion-trap alignment and when the lens electrode I9 is operated at a potential substantially equal to that of the cathode I3.

Merely by way of illustration and in no sense by way of limitation, it may be desirable to tabulate certain critical dimensional relationships in an operative embodiment of the invention constructed in the manner shown and described in connection with Figure 1. Satisfactory results have been obtained with a tube of the type shown in Figure 1 having the following dimensional relationships Angle of gun tilt 4 45' Length of grid 3 millimeters 30 Lateral offset of grid 3 with respect to grid 2 inches .063 Longitudinal spacing between grid 2 and grid 3 inches .080 Diameter of aperture in grid do .040 Diameter of aperture in grid 2 do .075 Diameter of aperture l8 do .080 Diameter of aperture in grid do .080 Inner diameter of grid 4 do .500 Inner diameter of corona rings 3| and 32 inches .470 Axial length of grid 4 do .125 Diameter of rollover of inset 33 of grid 4 inches .045 Axial spacing between grid 3 and grid 4 inches .110 Axial spacing between grid 4 and grid 5 "inches" .110

The dimensions of grids 3, 4 and 5 and the spacings between these electrodes are quite critical if optimum focusing conditions are to be achieved 8 with a focusing voltage substantially equal to that of the cathode. It has been determined that uniform results may be obtained by maintaining these critical dimensional relationships within a 53 manufacturing tolerance of plus .001 inch, minus .000 inch.

Certain features of the image-reproducing device disclosed in Figure 1 are particularly disclosed and claimed in the copending application of Constantin S. Szegho, Serial No. 229,013, filed May 31, 1951, for "Image-Reproducing Device, and assigned to the present assignee.

While the invention has been shown and described in connection with an embodiment employing an ion-trap type electron gun, it is apparent that the novel focusing electrode construction of the present invention may be employed to advantage in a unipotential electrostatic focusing lens system for a cathode-ray tube in which no ion trapping is accomplished in the electron gun, as for example in a cathode-ray tube employing an aluminum-backing layer for the fluorescent screen. The novel construction is particularly advantageous in connection with a low-potential electrostatic focusing lens system of the unipotential type in which the maintenance of stringent manufacturing tolerances is of critical importance.

While a particular embodiment of the present ."o invention has been shown and described, it is apparent that various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. In a unipotential electrostatic focusing lens system for a cathode-ray tube, an electrode comprising: a metal disc having an aperture; and a 4 metal inset having a cylindrical central portion within said aperture and having an outwardly rolled rim at each end overlapping and terminally abutting said disc. I

2. In a unipotential electrostatic focusing lens system for a cathode-ray tube, an electrode comprising: a metal disc having a circular aperture which is large relative to the thickness of said disc; and a metal inset having a cylindrical central portion externally engaging said disc within said aperture and having an outwardly rolled rim OTHER REFERENCES Industrial Electronics and Control by R. G. Kloefller (U. S. Patent Office libr.), copyright 1949, publ. by J. W. Wiley & 00., p. 453, Fig. 2. 

